Did Our Primate Ancestors Help Make a Murderer?

making_a_murderer_titlecardIf you are a fan of crime-drama shows, then you are probably aware of the NetFlix series Making a Murderer, which covers the story of  Steven Avery. After being exonerated of committing a rape by DNA evidence, Avery was convicted of the brutal murder of a female photographer. The show follows attempts of Avery’s lawyers to prove his innocence and that the local police had manipulated evidence to make Avery appear to be the murderer.

So what does make a murderer? Scientists are beginning to answer the questions as to whether murder is something that is a byproduct of human culture (our environment), or whether there some biological aspect (our genes) that makes our species inherently violent.  While there have been a number of studies that have explored the genetics of human aggression, it is interesting to wonder if violent behavior, especially murder, is something that we inherited from our primate ancestors. Are we, as humans, more violent than our ancestors?

An Evolutionary Perspective

spider_monkeyIn order to understand these questions, it was necessary to look at violent behavior in our evolutionary past. That is exactly what a team of scientists from the University of Granada did. The premise was relatively simple, if there is an evolutionary basis to violence, then species which have a close evolutionary relationship should have similar levels of lethal violence. To test this hypothesis, they looked at evidence of lethal violence behavior in  1,024 mammal species, including everything from primates to whales. As you might expect, many mammal species (over 40%) do not display evidence of this form of violence, but among those that do, the values of closely-related species indicated that there may be a relationship. Furthermore, social factors clearly influenced the level of lethal violence. Social structure and territorality of the species had the ability to moderate the predicted level of violence. Generally speaking – the more social and territorial, the higher the level of lethal violence.

This data was compared to information obtained on 600 populations of humans from 50,000 BCE till modern times. Interestingly, early human hunter-gatherer societies displayed levels of lethal violence that were comparable to our primate (chimpanzee) relatives, suggesting to the researchers that there was an evolutionary factor involved.

medieval_battleDon’t Rule Out the Environment

As you may expect, the level of violence in human in human history has fluctuated wildly. We have gone through some rough times as a species, and there have been periods of time where lethal violence was much more prevalent than today.The researchers decided to take a look at the social-political structure of the human populations to identify a pattern. They divided the populations into groups – bands, tribes, chiefdoms and states – and then analyzed the level of violence.  In the earlier periods, the level of lethal violence was about 2 percent, the same as is found in our primate relatives. But as society developed into warring tribes and chiefdoms, these values fluctuated as high as 30%. Today, modern societies have levels of lethal violence that are far lower than what is predicted by our evolutionary past, suggesting that the environmental structure of modern society is able to modify the biological influence.

Not Everyone Agrees

But as you would expect in science – not everyone agrees with this study. Not only are there problems with identifying evidence of violent behavior, such as murder, in the past, but there is also the question of what was classified as a murder. For example, some primates regularly practice infanticide, but have very little evidence of adult-adult homicide. The scientists themselves point out some of the limitations of their study and suggest that it primarily should serve as a database for additional studies. According to José María Gómez, the lead author of the paper, “”Our study provides a detailed phylogenetic and historical context against which to compare levels of lethal violence observed throughout our history”.

The Take-Home Message

So what does then make a murderer? While we have an evolutionary history of violence in our species, the level of that violence is modified by the social-political structure of human society.   According to Dr. Gomez,  “The main message of the study, from our point of view, is that no matter how violent or pacific we are in the origin, we can modulate the level of interpersonal violence by changing our social environment.” And perhaps that is the real finding of this study, that we are beginning to understand some of the factors, both good and bad, that make us human.


Additional Information


Michael Windelspecht is the primary editor and writer on Ricochet Science. He has a PhD in evolutionary genetics from the University of South Florida and has taught introductory biology and genetics for almost 20 years. He is also the managing/lead author for the Mader/Windelspecht series of introductory biology texts. For more on Michael, visit his website – michaelwindelspecht.com

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